The use of optical fibers as light-conducting means, long regarded as having a great future in numerous applications (telecommunications, links between computers, etc.), is currently experiencing a considerable expansion, notably by reason of the appearance on the market of low-loss fibers.
However, there still exist at present a certain number of problems which militate against the general use of optical fibers, a major one of which relates to the injection of light into the core of these fibers. It is in fact well known that optical fibers with planar end faces have a relatively narrow light-acceptor angle, so that in general these fibers can collect only a small fraction of the light beam to be injected, especially in the case of the beam emitted by strongly divergent light sources such as laser diodes since the core of the fiber can only accept those of the rays which arrive at an angle smaller than or equal to its acceptor angle. In other words, this inherent limitation of the optical fibers having planar end faces leads to the major drawback of relatively low efficiency of source-fiber couplings.
A number of solutions have already been proposed in attempting to improve this coupling efficiency; they are essentially based on the interposition of optical adaptor elements between the source and the fiber. Some of the solutions thus proposed consist in using independent optical elements located at a distance from the fiber and/or from the source (for example the use of a pair of orthogonal cylindrical macrolenses, or the use of a transverse fiber as a cylindrical microlens). However, the use of independent optical elements is fraught with the serious drawback that, by increasing the number of the elements forming the coupling system as a whole, it magnifies greatly the mounting and assembly problems of this coupling system and in particular the problems of high-precision positioning and alignment of the different elements relative to one another. This is the reason why, at present, the trend is towards the concept of optical elements integrated at the extremity of the fiber, so as to minimize assembly problems. The design of such integrated elements in turn raises a certain number of other problems concerning their manufacture and/or their integration with the fiber as well as the matter of their optical performance.
To realize such integrated elements, it has for example been proposed to join microlenses of semicylindrical shape to the extremities of the fibers. However, such a solution gives rise to serious problems in the manufacture of these semicylindrical microlenses as well as in the attachment of these microlenses to the fibers. In an attempt to remedy these drawbacks, it has accordingly been proposed to produce these microlenses directly on the extremities of the fibers, by using fusion methods. The microlenses thus produced on the fiber extremities, although easy to manufacture, yield only a rather mediocre optical performance, by reason of the slight curvatures obtainable in this manner whose control is moreover greatly limited by the differences in the melting temperature of core and cladding of the fiber. To avoid the above-mentioned problem, it has then been proposed to remove the cladding of the fiber by chemical attack before proceeding with the production of the microlens on the extremity of the core thus laid free. However, this solution is far from satisfactory inasmuch as the eventual increase in coupling resulting therefrom risks to be canceled out completely by the increase in optical losses inherent in the removal of the cladding.
Furthermore, it has been proposed to produce directly on the extremities of monomodal fibers cylindrical or hemispherical microlenses of ultra-small dimensions (radius below 5 microns), made of a photoresist material, by forming these microlenses by means of microlithographic methods. However, microlenses of this kind can only slightly increase the coupling efficiency, in view of their ultra-small dimensions which approximate to a dangerous extent the wavelengths of the light rays to be collimated (the ultra-small dimensions being capable of bringing about diffraction phenomena which may annul completely the desired collimation effect). Moreover, the formation of such microlenses could not be extended to multimodal fibers, since the methods which make this manufacture possible (injection of UV rays at one of the extremities of the fiber to bring about the polymerization of the photoresist covering the other extremity) remain exclusively limited to monomodal fibers. In addition, the photoresist material constituting these microlenses will probably give rise to problems of long-term chemical stability, which in turn would imply a strong restriction of the service life of the coupling systems equipped with such microlenses. Finally, it is very doubtful whether the manufacturing methods employed for producing such microlenses would make it possible to obtain lens surfaces which would be perfectly satisfactory from the optical point of view.